Wire bonding processes are frequently used in the fabrication of integrated circuit dies in order to form interconnects between a die having integrated circuits and a pin of a device package. Furthermore, it becomes more and more desirable to use metal interconnects based on copper (Cu). However, it is technically difficult to use Cu wire bonding for logic products as, for example, the forces occurring throughout the bonding process may be so strong that underlying chip structures may be damaged.
For example, when using Cu wire bonding on thin Cu pad metallizations (having, for example, a thickness of about 5 μm) the occurring forces may lead to cracks in the metallization (metallization cracks) or damage of the active structures below the bond wire (e.g. cratering). Due to the greater hardness of Cu metallizations compared to aluminum (Al) metallizations high values of the bond parameters (e.g. ultrasound, force, time) are generally required for welding the wire material onto the Cu chip metallization. Due to the high values of the bond parameters the mechanical stress may be increased in the metallization under the bonding site and in the underlying active structures.
Thus, damages in the form of cracks may occur in the top metallization or in the metallization layer arrangement (Cu, TiN/Al) or damaging of devices (e.g. transistors) under the metallization may occur. This may lead to total failure of the semiconductor device or to failures after a long time of operation of the device.
Another issue with Cu wire bonding is that oftentimes only a small part of a Cu pad will be contacted by the Cu ball in a ball bonding process.
One approach to address the above-mentioned issues is to use Cu based interconnect structures that have thick pads or large pads in combination with large Cu wires. These structures may be relatively stable (e.g. with respect to crack formation) but also complex and/or costly. Furthermore, a structure with large pads and large wires may not be suited for logic products with fine pitches.
Other approaches to address the above-mentioned issues are tuning of the bond parameters or cleaning the Cu surface from oxidation.